Different methods are known for converting frequencies into a digital word.
One method is the "counting method": a counter counts the edges of a frequency to be measured during a predetermined period of time. This time (gate time) period is determined by means of a reference frequency. To this end, a predetermined number of cycles of this reference frequency is counted.
Another method of frequency-to-digital converting is the "time method". In this method, within one gate formed by one or several cycles of the frequency to be measured, the time is determined by counting a reference frequency.
Finally, a combined counting and time method exists. In this method the edges of the frequency to be measured are counted according to the counting method. After the gate time has elapsed the remaining period of time till the beginning of the next full cycle of the frequency to be measured is determined.
The known methods are, according to the prior art, constructed with a plurality of discrete commercial components, such as counters. These components require much space. The production is complicated and expensive. The reliability of such a frequency-to-digital converter is low due to the large number of different components. Furthermore, it is difficult to test such a frequency-to-digital converter.